Bromelain is a mixture of proteinases derived from pineapple stem that is marketed in health food stores as a "digestive aid". Orally administered bromelain was anecdotally reported to induce clinical and endoscopic remission of ulcerative colitis in two patients whose disease was refractory to multi-agent conventional medical therapy. However, the potential efficacy of bromelain in colitis has not yet been tested rigorously in either animals or humans. In this study, the clinical and histologic severity of inflammatory bowel disease (IBD) was determined in IL-10-/- mice treated orally with bromelain in vivo. Daily treatment with oral bromelain beginning at age 5 weeks decreased the incidence and severity of spontaneous colitis in C57BL/6 IL-10-/- mice. Bromelain also significantly decreased the clinical and histologic severity of colonic inflammation when administered to piroxicam-exposed IL-10-/- mice with established colitis. Proteolytically active bromelain was required for anti-inflammatory effects in vivo. Adverse effects of dermatitis, hair loss, and weight loss due to mucositis were rare, dose related, and were not seen in wild-type mice treated orally with up to 1000 mg bromelain/kg/day for 18 weeks. Although the exact mechanisms by which exogenous proteinases affect bowel inflammation have not yet been determined, the results justify additional studies of this complementary biologically based approach to treatment of IBD.